An ova and parasite (O&P) exam is a microscopic evaluation that is used to look for parasites that have infected the gastrointestinal tract. The parasites are shed from the gastrointestinal tract into the feces. When thin smears of fresh or preserved stool are put onto glass slides and stained, the parasites and their ova or cyst (the form in which the parasite is surrounded by a resistant cover or capsule) can be detected and identified under the microscope. Different ova and parasites have distinct shapes, sizes, and internal structures that are characteristic of their species.

There are a wide variety of parasites that can infect humans. Each type of parasite has a specific life cycle and maturation process and may live in one or more hosts. Some parasites spend part of their life in an intermediate host, such as a sheep, cow, or snail, before infecting humans. Some infect humans "by accident." Most parasites have an adult form and a cyst/egg/ova form. Some also mature through a larval phase, which is between the egg and the adult. Ova are hardy and can exist for some time in the environment without a host.

The majority of people who are infected by gastrointestinal parasites become infected by drinking water or eating food that has been contaminated with the ova. This contamination cannot be seen; the food and water will look, smell, and taste completely normal. Since an infected person's stool will frequently contain ova, it is also a source of infection. Without careful sanitation (handwashing and care with food preparation), the infection may be passed on to others. This is especially a concern with infants at day care centers and the elderly in nursing homes. In these populations, a parasitic infection may be easily spread, and the immune systems of those infected may be less effective at getting rid of the infection.

The three most common parasites in the United States are single cell parasites: Giardia lamblia, Entamoeba histolytica (E. histolytica), and Cryptosporidium parvum. The Centers for Disease Control and Prevention (CDC) estimate that there are about 2 million giardia infections in the United States each year and that cryptosporidium (commonly called crypto) is the most common cause of recreational water-related disease outbreaks. Found in mountain streams and lakes throughout the world, these parasites may infect swimming pools, hot tubs, and occasionally community water supplies. Cryptosporidia resist chlorine and can live for several days in swimming pools. In the U.S., most parasitic infections are due to these three, but other parasites such as roundworms or tapeworms do occasionally cause infections.

Those who travel outside the U.S., especially to developing nations, may be exposed to a much wider variety of parasites. In warm climates and places where water and sewage treatment are less effective, parasites are more prevalent. Besides giardia, crypto, and E. histolytica, there are also a wide range of flat worms, roundworms, hookworms, and flukes that can affect the gastrointestinal tract and other parts of the body. Visitors usually become infected by eating or drinking something that has been contaminated with the parasites' ova - even something as simple as ice cubes in a drink or a fresh salad - but some of the parasites can also penetrate the skin, such as through the skin of the foot when someone is walking barefoot.

The most common symptoms of a parasitic infection are prolonged diarrhea, bloody diarrhea, mucus in stool, abdominal pain, and nausea. Patients may also have headaches and fever or few or no noticeable symptoms.